Aveline - you say that he's happy enough with the neighbourhood cats but are you sure there's nothing else in the garden at night? Fox? Badger? Something has set him off and a predator such as a fox could be enough. Any chance of setting up an outdoor/security camera to see what - if anything - is happening outside?
Other thing I wondered - you said this wasn't happening previously - were your daughter/husband working from home and have now returned to work? If the dynamic of the household has changed that could also be part of his issue.
What did you you think you would have by your current age that you don't?
Only 50% of middle age adult manage more than 1 brisk 10 min walk a month.

